(1) Field of the Invention
This invention relates to an acoustic imaging system and more particularly, to a system and method for acoustic imaging of the contents of an underground tank.
(2) Brief Description of the Prior Art
Monitoring the contents of underground tanks such as radioactive waste tanks, has always been difficult and problematic. Such tanks are buried underground allowing the soil to provide extra shielding from radiation. Such tanks also tend to be at remote sites for the obvious purpose of minimizing human exposure to radioactivity.
Radioactive waste may contain both radioactive and inert material. Monitoring such waste tanks typically includes measuring the internal distribution of radioactive material or sediment as compared to inert material. Prior art methods include removing and analyzing-a core sample of material obtained from the tank. The core sample is dangerous to manipulate, requiring special handling and analysis. Because each sample only represents a single point in time, samples must be continually taken to track the distribution and breakdown of radioactive materials over time.
Sound waves can be used to analyze the contents of an area using acoustic imaging. One technique is to situate a transducer at the top of such a tank and transmit acoustic waves into the material, and analyze the reflected wave. This technique can be used to obtain an image of the contents of the tank including the level of sediment in the tank.
Placing a transducer on top of a tank, however, will not work in most situations. Because of gravity, material in tanks tends to settle and stratify into horizontal layers. If some layers contain gas bubbles or are otherwise impervious to acoustic wave propagation, it is not feasible to propagate sound perpendicularly through these layers. Accordingly, what is needed is a system and method for propagating sound horizontally through the side of an underground tank.
One prior art method involves providing an array of vertically arranged transducers next to the tank. Situating such a vertical array to the side of a tank is also problematic, particularly for previously installed tanks. The transducer array must be in good acoustic contact with the contents of the underground tank. Either the transducer array must be installed inside the tank, which requires an opening and the danger of breaching tank integrity, or the transducer array must be installed against the side of the tank. Drilling into the soil next to the side of the tank is extremely dangerous, not only for the danger of breaching the radioactive containment tank, but also due to the possibility of radiation exposure to persons working close to the tank.
Accordingly, what is needed is a system and method for acoustically imaging the contents of a remote tank, which can provide real time data, and poses no danger of radioactive waste tank breach.